Police hunting for a 19-year-old suspect who boasted online about the killing of a nine-year-old boy have searched a hospital in western Germany, but came up empty-handed.

A witness had tipped off police, saying he had seen suspect Marcel Hesse at a hospital in Moenchengladbach, 80 kilometres (50 miles) south-west of Herne, where the boy was killed on Monday night, the dpa news agency said.

After their investigation, authorities found that the person spotted was not Hesse, police said.

Authorities have said Hesse posted boastful pictures of the boy's body, clearly showing stab wounds, on a web forum and may have injured himself in the hand.

A nationwide manhunt for Hesse was launched on Tuesday.

Police have described Hesse as potentially armed and asked people not to approach him if they see him.